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Is there anyone manufacturing new DME devices for small GA planes? (and GPS substitution)

Michael

It is your attitude that is the problem, not using GPS distance substuted for DME, if you cant see the possibility for error then you are not likely to do the required cross checking to counter those errors.

Highly automated systems require careful crosschecking to avoid errors, most GA aircraft are flown single pilot so a second pilot checking the navigation flight plan is a luxury you don’t have to catch any errors you make. This makes it essential that the single pilot is fully prepared to be open to the possibility of these errors.

In the USA they have as you say been doing this for some time and no doubt with feedback the approach designers have changed things on the approach design to as far as posable to avoid such problems as dual NAV aids with the same indents creating the likelihood of distance errors, this will not have happened yet in Europe because GPS for DME has not been approved until then using the DME will be the safest option.

I’ll start by answering the last question :

DME accuracy is generally considered to be 185 m (±0.1 nmi) .

GPS WAAS is 7.6 m

Now we can start to talk about DME=0 nm vs ARP, for example…

FAA A&P/IA
LFPN

I like the fact that having DME means flying an ILS with DME is easy. Of course GPS can be used instead but it can lead to problems if you aren’t careful.

I also like that i can easily have a distance separate from whatever GPS waypoint i am heading for.

And for the avoidance of doubt I am the sort of person who loves GPS and asks for the GPS approach when assigned the ILS.

Last Edited by JasonC at 30 Nov 21:49
EGTK Oxford

One human factors decision making model uses a three legged stool. The idea is that if any leg is broken the stool cannot be used.

So if we consider the stool to have 3 legs…..

Is it legal? ie your licence can be removed
Is it safe?
Is it advisable?

We see that irrespective of whether it is safe or advisable it is ILLEGAL.

To plan to do something illegal is reckless.

Doing something illegal because it was the only way…perhaps to save life…might be explainable in a court of law.

In my oppinion just because a blind eye is being shown to Cirrus aircraft, does not give it a green light.

As I understand it the legal situation allows you to do some ILS approaches without a DME, but only if the approach specifically states that…in those cases a DME fix by flying over a VOR or getting a ground radar fix is specified on the plate. But these approaches are rare and I don’t know of any.

So when will this requirement be changed to fit in with the FAA rules?

In lieu of the Scotland accident, I suspect no time soon.

Hampshire

@Michael, your last point is completely irrelevant. The display precision is 0.1 miles, so maximum accuracy is 0.05 in both cases (or “within 0.1 mile”, if you prefer). And if you use a GPS waypoint that is a mile off, it does not matter if you fly into terrain with 7m or 180m precision.

The problem is human factors. People screw up. Using an approach with a good chart with exactly the aids as designed (including using an overlay in the Database, and including substitution of criminally inaccurate NDBs even if no overlay is present) minimises potential for screw up. DME substitution – in some cases – increases potential for screw up, and I prefer to minimise it. At least one pilot didn’t and died as a result. I don’t want to be the next one.

I agree with you that we should be perfectly entitled to do GPS substitution. We can also all minimise the risk even then by not flying any approaches where substitution is non-trivial, or where there is no cross-check.

But please do not attack pilots that have a different attitude to risk, or a more humble opinion of their fallibility.

Biggin Hill

To plan to do something illegal is reckless

In aviation, but in particular in Europe, regulations have stopped being a sensible guide to what is safe long ago. I plan to illegally fly ndb approaches using the overlay and no ADF on board, and would illegaly fix an autopilot by replacing a dried out capacitor instead of removing it and fly without it.

The much more sensible appoach of the FAA clearly demonstrates that it can be safe enough.

Cirrusses without DME should not only be tolerated, but completely legal. Operating without the DME is marginally less safe,and the degree of safety is the same whether it is legal or not.

Last Edited by Cobalt at 30 Nov 22:14
Biggin Hill

Cobalt wrote:

And if you use a GPS waypoint that is a mile off, it does not matter if you fly into terrain with 7m or 180m precision.

Know of any DME approaches where the DME station is a mile off ???

FAA A&P/IA
LFPN

Cobalt wrote:

Cirrusses without DME should not only be tolerated, but completely legal.

Cirrus are not the only ones not delivering DMEs as standard. My Lancair Columbia never had one and none of my clients’ late model Ovations have DME either.

Cobalt wrote:

Operating without the DME is marginally less safe,and the degree of safety is the same whether it is legal or not.

What makes you think that it’s “marginally less safe” ?

Is that an opinion or based on hard facts ?

FAA A&P/IA
LFPN

I like the fact that Michael is willing to tackle the GPS-for-DME-is-dangerous old wives tale head on, but I fear that most people are not open minded enough to consider that he might have a point.

1) Clearly if using GPS in place of DME, you enter the identifier for the DME itself, NOT the airfield identifier! As the DME may be off the airfield. Doing this correctly is no more error prone than selecting the wrong DME frequency. This is all the Dundee guys needed to do.

2) This just leaves DME bias (a zero reading at a non-zero distance from the transmitter) plus slant range as remaining two error factors on approach. The DME bias gives more safety margin if not implemented in the GPS, as it reports you as further than charted and hence you will be too high on the approach not too low. The slant range difference puts you nearer (and lower) if not implemented in the GPS, so theoretically less safe but in practise the difference is negligible at approach altitudes.

3) On the missed approach you might start a turn too early on GPS if the DME has bias, but you will be at safe altitude by then anyway.

I am still waiting for someone to show me an approach plate where using GPS instead of DME (with the DME’s identifier entered as the GPS waypoint NOT the airfield identifier) would be unsafe.

Restating my earlier post, it would be easy for Garmin et al to have a separate data block showing “derived DME” or some such from a tuned DME/VOR in Nav 1 or Nav 2…. no calculations required!

Last Edited by AnthonyQ at 01 Dec 00:05
YPJT, United Arab Emirates
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